This invention relates generally to the etching of openings in polyimide layers, and more specifically to the etching through a polyimide layer of openings having sloped side walls and a carefully controlled size and to the application of such etching to the fabrication of semiconductor devices.
In the fabrication of semiconductor devices of high density and high complexity, it is necessary to shrink the size and spacing of device features. It is also necessary or desirable to use thick layers of insulating material to isolate between the semiconductor substrate and an overlying metal layer or between multiple metal layers. It is further desirable that this thick insulating layer also act to planarize the surface upon which a subsequent metal layer is to be applied. Polyimide layers in which a liquid material is applied to the surface of the semiconductor substrate and subsequently heat treated to cure or form the polyimide material are well suited to the function of forming a planarizing and insulating layer. The use of thick polyimide insulating layers, however, especially on small geometry devices, presents a problem with metal continuity into contact openings. To provide the desired electrical contact between layers, contact openings must be formed through the polyimide material. Metal is then applied over the surface of the device and into these contact openings. Because the polyimide may be thick, however, it is difficult to achieve reliable metal coverage on the peripheral walls of the contact opening to the underlying material. Metal step coverage problems are enhanced by the need for the metal patterns to consist of narrow metal stripes which, in turn, require these stripes to be thin to allow fine geometry patterning. To ensure reliable step coverage into openings through the thick polyimide layer, it is imperative that the edges of the opening be tapered. It is also important, however, that the size of the opening be carefully controlled. The latter requirement stems partly from the fact that the device region being contacted may also be small. If the size is not controlled and the opening through the polyimide material is too large, then the subsequently applied metal may cause shorting between adjacent device regions. There are a number of processes which provide for either the tapering of the walls of the opening or for the control of critical dimensions, but there has not been a production compatible etching process which simultaneously achieved both of these necessary features.
It is therefore an object of this invention to provide a process for etching an opening through a polyimide layer wherein the opening has tapered side walls and a controlled size.
It is another object of the invention to provide an improved process for fabricating a semiconductor device including the etching of contact openings through thick polyimide layers.